Indirectly heated cathode



May 19, 1936. B. ERBER ET Al.

INDIRECTLY HEATED CATHODE Filed Oct. 29,l 1930 Patented May 19, 1936 INDIRECTLY HEATED CATHODE Bernhard Erber and Ma and Ferdinand Gantn signors to the iirin Gu Austria Application October 29,

. In Austria rgaretlie Furcht, Vienna, er, Modling, Austria, asstav Ganz.& Co., Vienna,

1930, Serial No. 492,020 July 3, 1930 13 Claims. (Cl. Z50-275) This invention relates to indirectly heated glowing or hot cathodes for discharge tubes. The indirect heating of discharge tub'es permits of the employment of electric currents of any kind and property, and therefore also of the employment of any sources of current, irrespective of the operation of the tubes. It has already been proposed to make tubes of the described order for operation with the voltages actually at the dism. posal of the user, such as the normal line voltages.

The proposals made in this direction have not been. put into practice for the reason that the results Vachieved thereby, considered from the vpoints of view of economy, length of life, and l; time required for starting, do not attain the standards expected of the systems of indirect cathode heating at present generally in use, in which systems the low voltage heating current is either derived from special current sources, or is produced by transformation of the line current.

The present invention has for its object to provide a method of indirect cathode heating with line'current which shall be free of the abovementioned drawbacks. To this end a heating wire consisting in the main of metal and having this great length which is necessary for consuming the high voltage with a wire of such material is so arranged within a body of insulating material, in utilizing the cross-section of this body, which in its turn is surrounded by the cathode, that the heat output amounts to at least 1-11/2 per sq. cm. of effective cathode surface.

No alteration in the usual structure of thedischarge tubes is necessitated by the introduction of this new system. IIt is however necessary to take special measures to accommodate the long length of heating wire used within the space available inside the cathode. This object is achieved by spiralizing the wire in a manner known per se, and by concentrating'the coil thus obtained within the insulating body situated inside the `cathode by laying the same in folds in meander or serpentine formation.

With this arrangement the heating wire need only be heated to a low temperature, a fact which is of great importance for the length of life of the tubes. At the same time only comparatively slight diierences of voltage occur between the individual sections of the heating wire, so that, for the prevention of short-circuiting between these sections, it issumcient to make the insulating layers between these sections as thin as is practically possible. This yfact results in the further advantage that the heat capacity of -the u' cathode body is very slight, and, with the employment of the usual small amount of energy. the cathode attains its working temperature after a comparatively short starting interval. At the same time it becomes further possible to arrange the heating wire in such a concentrated forma- -5 tion that the high voltage applied thereto produces no disturbing eiects either 'of a capacitive A or of an inductive order.

This arrangement proves not only particularly effective in regard to the accom considerable length of wire, required, but also permits, of an economical distribution and concentration of the heat generated, and thusproduces very favourably heating conditions. If this circumstance be fully utilized the temperature of l5 the heating wire can be kept down to a minimum which is very little higher than the temperature required for the glowing cathode itself, the dimensi-ons of the wire being so determined within the limits set on the one hand by the voltage employed and on the other hand by the permissi`1 ble heating limit for the wire under prolonged working that the heating is effected at precisely the temperature which suflices, in the case of the particular method employed for distributing or disposing the` wire, to maintain the cathode at the desired temperature. The useful life of the wire is thereby considerably increased.

In' some cases it has proved advantageous in accordance with the present invention to make the hot cathode of a discharge tube of several independent parts which are connected in paral lel, while the sections of the coiled heating wires which pass through these parts are connected in series. It is possible in this favourable conditions of emission and heating even with heating currents of very high voltage.

'It is further well known that in the manufacture of hot cathodes the heating of the cathode to a very high temperature is necessary in the course 40 of the so-called baking and ageing processes. If the heating wire according to the present inve tion were heated to temperatures of this order its life would be considerably shortened thereby, and one of the valuable achievements of the present invention, namely the increase in the length of life of the heating wire, would be sacrificed. In order to avoid this loss the invention provides for the incorporation in the cathode, apart from the coiled heating wire, of an Ordinar wire which can be provided with separately vcurrent conducting leading in wires, and is so dimensloned that with the usual low voltage the required high temperature of the emitting layer (about 12001400 C.) during the process of 55 modation of the 10 manner to obtain 35 y simple heating y manufacture can be attained. After the completion of the lamp this latter wire becomes superiluous; it is therefore permissible to overheat the same considerably, since it is only required to function for a short length of time. This wire can, however, be so dimensioned that it can serve fox` the auxiliary or alternative heating of the tube under normal working conditions with low voltages, so that it thus becomes possible, for example, to have a tube which can be heated both by current from the main and also by current from accumulators. v

The heating required during the process of manufacture (for the baking of the tube or for the ageing at the cathode) can also be carried out in another manner which does not entail putting a strain upon the heating wire. In place of a metal tube with a coating oi' an emitting medium the cathode proper can also consist of a similarly coated wire which is wound into a spiral and through which current can bel directly passed for the purpose of the initial heating at high temperature. of the tube after completion is however effected indirectly by means of a heating unit in accordance with the present invention accommodated within the said spiral. In this case also the separate conducting-leads can be so arranged that the tube may be operated alternatively with two different voltages, that is to say with direct or indirect heating.

The main advantages of' the invention are first the simplification of the various kinds of apparatus in which tubes of this kind are employed, and secondly the rendering of direct and alter- -nating current of high voltage to be in the same manner applicable to the operation of these tubes. Whereas hitherto direct current sources or lines with a high voltage were precluded from use for the operation of tubes with indirect heating the application of the present invention enables both kinds of current to be employed with equal efiicacy also at high voltages in regard both to efficiency and to all other conditions of working.

Hot cathodes of the type provided by the invention can be employed wherever indirectly heated cathodes are used, but more especially in luminous and similar discharge tubes.

Several examples of the embodiment of the v invention are shown in the accompanying drawing, in whichz- Figs. 1 and 2 illustrate a glowing cathode in which a cylindrical tube coated with an emitting medium is heated bymeans of a spirally Wound and repeatedly folded wire each of the folds of which is enclosed in a separate insulating tube.

Figs. 3 and 4 illustrate a modified form of the invention in which the individual folds of the heating wire traverse channels in a ceramic insulating body which lls out the interior of the cathode. These figures also illustrate a manner in which an auxiliary wire can be inserted in addition to the heating wire.

Figs. 5 and 6 show a further modification of the cathode in which the emitting film is carried by a box-shaped carrier having two substantially predominating plane surfaces. In this case again the folds of the heating wire are laid in channels in a ceramic insulating body which fills out the interior of the box-shaped cathode.

Fig. 7 shows yet another modification of the cathode with an insulating body similar to that shown in Figs. 3 and 4, in which, however, the cathode proper consists of a spirallywound wire which is coated with an emitting medium.

'I'he normal heating for the operation Fig. 8 shows a still further modification. v, Referring to Figs. 1 and 2 the indirectly heated cathode shown consists of' a hollow cylindrical nickel tube a which carries a coating of an emitting medium which is heated by means of a heating wire b situated in the interior of this tube. This heating wire, as above explained, is of such a length that its resistance suflices, at comparatively high voltages, to generate the required output of heat with the desired not 'too high temperature. For the purpose of accommodating this length of wire within the space enclosed by the surface to be heated this wire is spirally wound for the whole of its length and in the illustrated example moreover. laid in four folds travelling back and forth within the cathode tube. Each fold of the wire is enclosed in a separate insulating tube c, and all four tubes are so disposed that they as nearly as possible uniformly fill out the hollow space within the tubular cathode.

The heating wire may itself be provided with a coating of insulating material in the manner of enamelled wire. For example, the coils can be provided with a coating of ceramic material by spraying it on the wire with a suitable binder to the required thickness and finally baking it, by which means the wire acquires a suitable coating which is so constituted, in the case of the multiple coils here employed, that it prevents at the same time the deformation of the coil. The coating may also consist'of pure glowed aluminium oxide. zirconia, or similar material which is applied in the same manner.

The modified form of cathode shown in Figs. 3 and 4 is distinguished from that described above in that the function of the separate insulating tubes is assumed by a single ceramic insulating body which completely fills out the interior of the tubular cathode. 'I'his insulating body is provided with a'number of channels (there being four such channels in the example shown in Figs. 3 and 4) which are traversed by the folds of the spirally wound heating wire.

That a sufficient length of heating filament of` the' appropriate thickness can be accommodated within the cathode is shown by the following specific example:-

The hot cathode is intended for direct connection to a lighting main carrying current of volts. The cathode is to consist of a nickel tube of approximately 3 mm. internal diameter and approximately 30 mm. in length (which corresponds to the usual dimensions). In this case a heating coil is employed consisting of tungsten wire of 0.02 mm. diameter. The total length of this wire required to effect the heating in accordance with the present invention is cm. This length of wire can easily be converted by winding into a spiral which, according to the diameter of the turns, measures 80-140 mm. from end to end. This length of spirally wound wire can easily be accommodated within the space enclosed by the cathode if the coil be folded at the most four times.

When current of 220 volts is to be used vprecisely the same coiled heating filament can be employed, two tubular cathodes being arranged in juxtaposition and connected in parallel, while the heating wires contained therein'are connected ly increasing the diameter of the tubular cathode to provide `the extra-space Vnecessary for all the required additional folds of the spiral (in the example cited) purpose of heating.

- 'I'he length;oftheheatingiilament,as already mentioned, can-be so determinedthat itsworking is in excess cf that strictly required by the voltage or by the highest permissible temperature for the wire under continuous working. In consequence of the uniform distribution of the heating wire in regard to the surface of the cathode, and in consequence of the concentration of the heating eiect hereobtainabla-and also in consequence of the favorable conditions for the transference of heat, it becomes possible through the present invention to maintain a minimum temperature of the heating wire which just suliices to bring the emitting layer up to the required temperature of 7001000 C., that is to say the wire itself does not require to beheated much beyond this temperature. Through the application of the present invention it also becomes possible, in conjunction with very thin wires, to develop a calorlic output which amounts to at least 1-11/2 watts per cm2 of cathode surface andis generally con- 'siderably higher. It is, however, only above this limit that the most favourable, i. e. lowest, temperature of the heating filament under working conditions can be obtained.

A further step in the direction of improving the economy and length of life of the tubes-by reducing as far as possible the temperature of the heating wire consists in providing that no more of the energy supplied to the heating wire is allowed to go to waste on the way to the emitting layer than is absolutely unavoidable in consideration of the required insulation between the heating wire and the hot cathode. The procedure is to arrange the heating wire and the metal of the cathode in as close proximity as possible while allowing for the necessary insulation, to which end the metal of the cathode is applied directly to the layer of insulating material surrounding the heating Wire, and in addition this layer of insulating material between the heating wire and the metal of the cathode is kept as thin as possible.

The proposal has already been made, in connection with cathodes intended for indirect heating with currents of low voltage, to apply the cathode metal directly to the insulating body surrounding' the heating element, but clearly not for the purpose of enabling the temperature of the heating wire to be reduced, since this temperature is practically of no consequence for the length of life of the thick wires employed in connection with low voltage heating. Within the scope of the present invention, on the other hand, in view of the fact that even slight reductions in the temperature of the heating wire can be of great importance, this measure is resorted to for the purpose of increasing very considerably the length of life of the tubes. The end in view is not fully achieved by means of this measure alone, but only through at thefsame time keeping down theinsulating layer between several sectionsof the heating Wire and between the heatmanner a carrier layer temperature of th ing wire and the metal yof the cathode to the.

least-permissible thicknes's.- l

According 12,0 this principle, vfor instance, the

channels inthe insulating'body c in the form of construction shown-inlli'igs.` 3 and 4 'are to be 5 sodi'sposed so that-as little as possible of the heat generated by theheating -wiresis wasted in heat- V ing up-the lnsulatingbody; "The `cathode metal I is thenA not, as previously described, tted overi l .the insulating body inthe form of a se aratel temperatureislkept-down to a minimum, .ii'itlrav` p y view to increasing as far as possible its length of life. In this case the total length of wire usedm'ade tube (which` would imply the presence of. an.-insulatingyclearance between the insulating body andthe cathode-metal), butis directlyapplied by-means of 'any one otrthaknown processes for this purpose. These'processes .include 1.5 e. g. the spraying of the metal with a `pistol-4 atomizer, vapourization, electrolytic depositing, the method of cathode-atomization, etc. In `this for the emitting medium 1s obtained which is intimately connected to and 20 therefore adherent to the insulating layer or body, and which can b e kept considerably thinner than the above-mentioned cathode tube, and for this reason, if for no other, requires a smaller amount of heating energy. If desired the thickness of the cathode metal layer can be reduced to the lowest limit at which this layer for-ms a coherent surface throughout.

A form of construction which is particularl advantageous for the attainment of y 30 a very low e heating wire is 'shown in Fig.

8, 1n cross-section, on a very much enlarged scale. In this case an independent ceramic insulating body is not employed, but th heating wire b r5 1s itself provided, in the manner already deo scribed, with an insulating coating-c which in its turn carries a coating a' of the cathode metal. '.I'o this carrier layer a' the emitting layer which 1s not shown in the drawing, is applied.

It will be clear that with a cathode heating element of this type the en the heating of the emitting to the layer practically without loss, so that the temperature of the heating wire does not require 45 to .be appreciably diierent from that ef the emltting layer. By means of this favourable arrangement the duration of the necessary heating-up period is also considerably reduced.

Theform of cathode shown in Figs. 5 and 6 50 comprises a box-shaped body which is particu-V larly suitable for use in connection with box shaped containers and tube structures. In this 4case a correspondingly shaped insulating body is provided inside the iiat rectangular cathode 55 casing, which body permits of the provision of a large number (in the present instance eight) channels hollowed out of the material of ther said body. 'Ihe coiled heating wire is then laid along a meandrous course traversing these channels alternately in opposite directions. A glowing cathode of this type is mainly intended for use in connection with particularly high voltages. 65 The high temperature of the glowing cathode required during the process of manufacture for the purpose of baking and ageing the cathode, which temperature varies between 1200 and 1400 C., is preferably not generated by the heating wire itself, in view of the detrimental eiect of this step upon the life of the wire which is other- Wise very considerably lengthened by the present invention. According to the present invention itis preferable to generate the heat required for this purpose by means of a separate auxiliary heating wire 'f which is passed through the tubular cathode a in addition to the heating wire proper (Figs. 3.and 4). This auxiliary wire may be of comparatively large diameter, and can be submitted to a very great strain since it is not called upon to function during the subsequent operation of the tube, and can therefore be shortlived. A wire of this kind can, in consequence of its dimensions, be brought by means of the usual low voltage of 5-20 volts to the temperature required for the rapid and convenient liberation of gas from the cathode. Moreover the ageing of tubes of this type can easily be carried out by means of the auxiliary wire in consequence of the rapidattainment of very high temperatures which is facilitated by the use of this wire.

The auxiliary wire can either be provided with separate leads-out or can be connected to the same leading outV points as the heating wire proper. In the latter case the auxiliary wire is rendered inoperativein any suitable manner after the conclusion of the ageing process.

In Figs. 3 and 4 the insertion of an auxiliary wire f of this type within a cathode which is filled out with an insulating body is illustrated. The in- In Fig. '7 a further form of the invention is` shown in which the cathode consists of a core wire h to which the emitting layer is appliedand which is wound to form a coil in which the consecutive turns are arranged near to each other. Within this coil there is provided as before an insulating body with channels for the accommodation of the folds of the heating wire. A separate auxiliary heating wire is not provided inthis case, since the wire of the coil forming the cathode proper is employed for the purpose of the initial heating, and is to this end taken to suitable leads. During the baking and ageing of the tube the oxide cathode is thus directly heated to the temperature required for this'process. The subsequent heating during the operation of the tube is then indirectly effected by means of a coiled heat' ing wire as already described. It is also possible to determine the dimensions of the wires in such` .a manner that the tube .can be heated at will either from the main or from accumulators or any other source of low voltage current, that is to say that the tube can be used either as a directly heated or as an indirectly heated tube, according to the voltage of the current available.

What we claim, is: v y o 1. An indirectly heated cathodefor electric discharge tubes, comprising in 4combination a cathode surface consisting of an auxiliary heater adapted to be heated while aging the cathode, a coating of an emitting medium applied toa part of said cathode surface, a heater within said surface adapted to. be connected directly with ordinary electric supply means and consisting of a plurality of helically wound metal filaments havtransmitting insulation interposed between said cathode surface and said heater and between adjacent fllaments. f

2. An indirectly heated cathode for electric discharge tubes, comprising in combination. a cathode surface, a heater within said surface adapted to be connected directly with ordinary electric supply mains and consisting of' a plurality of helically wound metal filaments having a diameter substantially less than 0.1 mm., said filaments being interconnected in series and being closely adjacent to each other, and a separate heat-transmitting insulation tube surrounding each one of said filaments.

3. In an indirectly heated cathode for discharge tubes, an insulating body, an electron emitting surface carried by said insulating body and surrounding the same, a plurality of elongated helically coiled conductors connectedvin series with each other and situated in the insulating body, said conductors consisting of a large number. of closely adjacent turns and being situated in close juxtaposition to said electron emitting element to heat the same, and a separate auxiliary wire situated in said insulating body.

4. In an. indirectly heated cathode for discharge devices, a heater of a diameter substantially less than 0.1 mm. having an effective resistance to develop a caloriflc output amounting to at least l-llA watts per cm2 of cathode surface at the operating voltage, an electron-emitting cathode surface and a carrier therefor surrounding said heater, and a small mass'of insulation for said heater serving at least in part to separate it from the cathode carrier and-placed in contact with the latter, the mass of said in sulation being so small that during the operation of the device the temperature of the heater may be kept substantially the same as that required for the cathode surface quickly to emit electrons when the heater is first connected with a high v voltage source.

cathode is substantially filled with the insulation with channels formed therein, and the heater in the form of a Wire is in a plurality of layers each held within a channel.

7. An indirectly heated cathode according to claim 4 and in which an auxiliary heater is provided Within the cathode.

8. In an indirectly heated cathode for discharge devices, a heater having an effective resistance to develop a caloric output amounting to more than l-l1/2 watts per cm2 of cathode surface at the operating voltage, an electron-emit# ting cathode surface `and a thin carrier therefor surrounding said heater, and a thin layer of insulation separating said heater from the carrier and placed in contact with the latter, the thickness oi the insulation and of the carrier being such that during operation suflicient heat is transmitted to the cathode surface when the heater is operated only slightly above the critical temperature of the cathode surface to start the device without undue delay.

9. In an indirectly heated cathode for discharge devices, an electron emitting element, heating means therein adapted to be directly connected to a 110 volt line, and solid insulating means separating the heatingmeans from the emitting element, said emitting element encasing and contacting with said insulating means and having a surface area of substantially 3 om.

10. An indirectly heated cathode comprising an electron-emitting element for use singly and in Hseries with one another in a plurality of electronic tubes, heating means within said cathode, and solid insulating means separating the heating means from. the emitting element, said emitting element encasing and contacting with said insulating means and having a surface area. of substantially 3 cm2, each of said emitting ele-u ments encasing and contacting with its respective age of at least 110 volts, the filament being loy cated within coherent insulating material, said electron-emitting element surrounding and in contact with said material and having a superiicial area oi substantially 3 om.

12. In an indirectly heated cathode for discharge devices, an electron-emitting element and a heater surrounded by -said electron-emitting element and consisting of a helically coiled lament of a diameter of substantially 0.02 mm. and of a length of substantially 1700 mm. which helically coiled filament is in the .form of a plurality of series connected sections Within solid insulating material, said electron-emitting element enclosing and contacting with said insulating material and having a surface of substantially 3 cm2.

13. In an indirectly heated cathode for discharge devices, an electron-emitting element, a helically coiled heating wire of a diameter of 0.02 mm. and of a length of 1700 mm., said Wire being arranged in a plurality of series connected sections extending through longitudinal perforations in a coherent insulating body, said electronemitting element enclosing and contacting with said body and having a surface of less than 3 cm2.

BERNHARD ERBER.

MARGARETHE FURCHT. FERDINAND GAN'I'N'ER. 

